The educational work of the Registry has continued with distribution of study sets, consultations mainly with NIH staff, preparation of book chapters and articles and participation in the nomenclature committee for mouse hematopathology. Additional cases of neoplasms in experimental animals have been obtained and are being entered into the Registry files, a process considerably improved by adoption of a new computer program. A set of polyoma-induced tumors in mice is among these new cases, which also include retrovirus-induced hematopoietic neoplasms and neoplasms of the endocrine system. Recently acquired Registry material has been used to start a program to evaluate cytokine and oncogene expression on the basis of immunocyto-chemical characteristics in nitrosamine-induced rat tumors. In addition, these tumors are tested for cell cycling (PCNA) and identification of structural proteins. An important part of this study is to develop a set of positive controls for rodent proteins in tissues, and to compare expression of these proteins in rodents with published data on their expression in human tumors. About 20 tumor types are being tested, with progress governed to a large extent by the availability of rat-specific antibodies that will work in formalin-fixed, paraffin- embedded tissues. Immunocyto-chemical identification of cell surface antigens on mouse hematopoietic cells in frozen sections has been very successful and has provided one part of a multiparameter study of mouse lymphomas, which is nearing completion. All slides stained immunocytochemically are incorporated into the Registry's files as additions to those stained histochemically. This addition considerably broadens the scope of the Registry material and is particularly useful in the preparation of study sets.